An Investigation of Aggregation Within the Melting Layer

Aggregation is an important microphysical process occurring within mesoscale convective systems and layer clouds. It decreases the total number of particles in the cloud and increases the size and mass of the collector particle, which in turn affects its terminal fall velocity. Aggregation also affects radiative transfer and cloud properties observed by radar. Although studies have shown that aggregation is occurring above the melting layer, the significance of aggregation and breakup within the melting layer has not been completely determined. Several studies have given conflicting results of the presence and significance of aggregation within the melting layer. This study examines the observed microphysical properties of the melting layer to identify whether the aggregation process is occurring and is a significant process. The results will be relevant to modeling studies and remote sensing retrievals. Twenty vertical profiles of the melting layer were collected using the University of North Dakota Citation II research aircraft during the Midlatitude Continental Convective Clouds Experiment (MC3E) and the Global Precipitation Measurement (GPM) Cold-season Precipitation Experiment (GCPEx). Each vertical profile is tested for horizontal homogeneity by comparing the total number concentration and the largest particle diameter against both latitude and longitude. Only the profiles that are considered horizontally homogeneous are considered in this study. Gamma distributions are fit to 5-second samples of the combined spectra from the Two-Dimensional Cloud (2DC) and High-Volume Precipitation Spectrometer version 3 (HVPS-3) probes. The vertical distributions of the gamma distribution parameters and the maximum particle diameters are analyzed to determine the presence and degree of aggregation within the melting layer. The effect of aggregation upon a bin melting layer model is also investigated.